Buffy tried to shoot Lucius a skeptical glance, but the truth was she imagined things between he and Ambrose were eventually going to come to a head at some point. She hadn't wanted them to, and she hadn't wanted to think that they'd end up exchanging blows and hurting, possibly killing, each other. She would've been sure it was her fault, and so she'd wanted to be there to personally make sure that it didn't happen at all.
She found it hard to protest or think of a logical argument that would stand up against what he was saying. So, she just kind of stood there and looked uncomfortable as he continued. And then he hit the nail right on the head.
"No!" Buffy protested, not wanting to give him anymore ammunition but not really being able to effectively deny it. He did know her, and he'd always been able to tell when she was lying. She wrapped her arms around herself, looking away and at the ground for a moment. "I-it's not any of your business," she tried instead, attempting to sound angrier instead of actually deny it or lie to him.